1. Field of the Invention:
The present invention relates to a fused refractory. More particularly, it relates to a high zirconia fused refractory product having high corrosion resistance which is especially suitable for a molten glass tank and furnace.
2. Description of the Prior Arts:
A fused refractory product has been usually obtained by casting a molten mixture of a specific composition of refractory materials which is completely melted in an electric furnace into a mold having a desired shape and annealing the molten composition to the ambient temperature to resolidify the composition. The fused refractory products have been well-known as high grade refractory products which are quite different from the fired or non-fired rebonded refractory products in view of the structure and the preparation.
The typical refractory products comprising ZrO.sub.2 as a main component among such fused refractory products are ZrO.sub.2 --Al.sub.2 O.sub.3 --SiO.sub.2 type refractory products. Certain high zirconia fused refractory products have been used.
The known ZrO.sub.2 --Al.sub.2 O.sub.3 --SiO.sub.2 type refractory products usually have a content of ZrO.sub.2 of about 33 to 40% by weight and have a structure containing baddeleyite (ZrO.sub.2) and corundum (Al.sub.2 O.sub.3) in a glassy matrix to have excellent resistance to a molten glass. In view of the content of ZrO.sub.2, the corrosion resistance is limited.
On the other hand, a high zirconia fused refractory product consisting essentially of ZrO.sub.2 crystal (baddeleyite) in the structure has been known but it has not been practically used, because the fused refractory product contains about 90% by weight of ZrO.sub.2 and the abnormal expansion and contraction at about 1100.degree. C. caused as the specific characteristic of ZrO.sub.2 can not be easily prevented and block cast bricks having no crack are not easily obtained.
The cast refractory products containing about 90% by weight or more of ZrO.sub.2 have been proposed in U.S. Pat. No. 3,519,448 and Japanese Unexamined Patent Publication No. 85610/1973 and No. 121012/1978. These refractory products have high corrosion resistance as high zirconia refractory products to be effective, however they have not been satisfactory in certain usages.
For example, the cast refractory products comprising on a weigh base; 50% to 98% of ZrO.sub.2, 1% to 29% of Al.sub.2 O.sub.3, 0.1% to 25% of SiO.sub.2, 0.5% to 15% of a rare earth oxide, 0 to 6% of P.sub.2 O.sub.5, 0 to 5% of an alkaline earth metal oxide and 0 to 4% of an alkali metal oxide have been disclosed in U.S. Pat. No. 3,519,448. In accordance with the proposal, the stabilization of ZrO.sub.2 has been attained by the incorporation of the rare earth oxide in the high content of ZrO.sub.2 as it is understood in view of the broad range of the content of ZrO.sub.2 of 50 to 98%. As a result of various studies, it has been, however, found the fact that the incorporation of the rare earth component forms hard glassy component to cause cracks when the glass matrix component is less or nothing, whereas the rare earth component does not contribute for the stabilization for ZrO.sub.2 by melting into the glass component when the glass matrix component is mush. If the rare earth component is much in view of contribution for the stabilization, the content of the glass matrix components is increased to deteriorate the corrosion resistance as high zirconia cast refractory product. In the high zirconia fused refractory product as the object product of the present invention, the rare earth component is not a desired component in view of said disadvantages and expensiveness of the component.
In Japanese Examined Patent No. 15689/1972, the object products having high content of Al.sub.2 O.sub.3 have been proposed. The incorporation of the component of Al.sub.2 O.sub.3 causes the hard glass matrix component. In the case of the high zirconia refractory product, the incorporation of much component of Al.sub.2 O.sub.3 disadvantageously may cause cracks.
In Japanese Unexamined Patent Publication No. 85610/1973, the refractory products containing 90% to 98% of ZrO.sub.2 have been proposed. As a matrix component, CuO or B.sub.2 O.sub.3 has been incorporated whereby certain coloring of the glass product is caused by using the refractory product as the glass furnace. Therefore, the refractory product is not suitable to use it as the glass furnace for a glass product such as a soda lime type glass product which should not be colored.
In Japanese Unexamined Patent Publication No. 121012/1978, the refractory products containing 92% to 98% of ZrO.sub.2 have been proposed. The products have not disadvantages of the cracks caused by the rare earth element, the lower corrosion resistance and the coloring of the glass product caused by the component of CuO which are found in the productions of Japanese Examined Patent Publication No. 15689/1972 and Japanese Unexamined Patent Publication No. 85610/1973, however the matrix consists of SiO.sub.2, CaO and MgO to cause hard matrix glass phase. It is difficult to obtain a large cast block having no crack.